The present invention relates to a developing method for developing positive-working offset plates employing waterless offset processes. In particular, the developing method is suited for photosensitive printing plates which are composed of a layer support, a photosensitive layer containing a diazonium salt polycondensation product, and an overlying ink-repellent crosslinked silicone elastomer layer, wherein the diazonium salt polycondensation product comprises recurrent A-N2X and B units which are mutually linked by bridge members, especially methylene groups, which are derived from carbonyl compounds capable of condensation, A being the radical of an aromatic diazonium compound capable of condensation with formaldehyde, and B being the radical of a compound which is free of diazonium groups and is capable of condensation with formaldehyde, particularly of an aromatic amine, a phenol, a phenol ether, an aromatic thioether, an aromatic hydrocarbon, an aromatic heterocyclic compound or an organic acid amide.
Plates of this type have been disclosed by U. S. Application Serial No. 943,034, filed Dec. 17, 1986, equivalent to prior, not pre-published German Patent Application No. P 35 45 204.8, wherein the photosensitive layer comprises diazonium salt condensation products according to U.S. Pat. No. 3,867,147 and U.S. Pat. No. 3,849,392, with a super-imposed layer of crosslinked silicone elastomer. The teaching of this Patent Application, hereinafter called A), is hereby incorporated by reference.
If these or similar types of printing plates are employed, the photosensitive layer is crosslinked in the exposed areas, which thereby are rendered insoluble in the developer. During development, the silicone elastomer and the photosensitive layer cannot be rubbed off. In the non-exposed areas, the photosensitive layer remains soluble, and during development the silicone elastomer and the photosensitive layer are rubbed off and the support or, respectively, the adhesion-promoting layer on the support is laid bare. When a plate treated in this way is inserted into a printing machine and inked, the ink adheres to the bare areas and is repelled by the silicone-covered areas. Printing can be performed without dampening solution.
With this type of printing plates, the developer must diffuse through the silicone elastomer layer and dissolve the underlying photosensitive layer in the non-exposed areas, and thus make possible the rubbing-off of the silicone rubber from these areas. Furthermore, the developer must cause the silicone elastomer to swell and be able to disperse the rubbed-off elastomer portions to prevent the formation of flakes. Only then can good resolution be achieved.
Developers for plates which are suitable for waterless printing are known, for example, from U.S. Pat. No. 3,677,178. This publication suggests to perform development with water, to which small amounts of wetting agents are added, such as Na-lauryl sulfonate, alkylphenyl ether, polyethylene glycol, trimethylolnonyl ether of propylene glycol and polyalkylglycol ether. With this developer composition it is, however, not possible to develop plates which, in particular, correspond to type A).
The developer for waterless offset processes which is described in German Auslegeschrift No. 16 71 637 (equivalent to U.S. Pat. No. 3,511,178) comprises a mixture of butyl acetate, propyl alcohol and water. Due to the different evaporation rates, mixtures of this type change very rapidly, and they also result in insufficient image resolution (see Comparative Example Cl). The butyrolactone also mentioned in this publication likewise results in an unsatisfactory development, in particular, if plates of type A) are employed (Comparative Example C2).
U.S. Pat. No. 3,894,873 suggests the use of paraffinic hydrocarbons, benzene, toluene, xylene, and the like for developers. The developers can additionally contain a preferred amount of less than 50% of halogenated hydrocarbons, alcohols, ketones, carboxylic acids or esters. However, they are intended for plates where the photosensitive layer comprises photopolymers that induce crosslinking with the adjoining surface of the silicone elastomer layer that must not be attacked in the unexposed state. Plates whose composition corresponds, for example, to type A) cannot be developed with pure isoparaffinic hydrocarbons. This is also due to the different principle of operation (Comparative Example C3).
European Patent Application No. 0,043,132 (equivalent to U.S. Pat. No. 4,378,423) also discloses a developer for photoadhesive layers, which must not attack the photosensitive layer. The developer comprises a predominant proportion of paraffinic hydrocarbons with an admixture of 2 to 20% of propylene glycol derivatives. It is also possible to add solvents, such as water, alcohols, esters, ethers, carboxylic acids, aromatic hydrocarbons and halogenated hydrocarbons, in quantities which do not attack the uncured photoadhesive layer. The developers prepared according to the teaching of this publication are inappropriate for development of plates of type A), because their mode of operation differs from that of A) (development and dissolution of the photosensitive layer) (see Comparative Example C4).
German Offenlegunsschrift No. 35 43 961 describes a developer for waterless offset printing plates, which comprises at least two organic solvents, a surfactant and at least 30% of water. This developer is suitable for plates which contain unsaturated monomers, photosensitive polymers, azido resins or zinc salt complexes of condensation products obtained from formaldehyde and a diazonium compound as photosensitive, photocurable layers. One of the solvents is intended to dissolve the photosensitive layer in the non-exposed areas and the other solvent causes swelling of the silicone elastomer. The following are mentioned as swelling solvents: alcohols, esters, ketones, ethers, aromatic hydrocarbons, halogenated hydrocarbons and carboxylic acids. The following are claimed as suitable second solvents: paraffinic hydrocarbons, cycloparaffinic hydrocarbons, aromatic hydrocarbons, aliphatic ketones, glycol ethers and alkyl esters of aliphatic carboxylic acids. If a developer according to the teaching of this publication, containing at least 30% of water, is used for developing layers of, for example, type A), the resolution in the reproduction of fine screen patterns is insufficient (Comparative Example C5).